Injector control for a selective catalytic reduction system

ABSTRACT

Systems and methods are provided for controlling the amount and timing of nitrogen oxides reductant injected during a given injection cycle into an exhaust system of a vehicle as part of a selective catalytic reduction system. The amount of reductant injected is determined by at least one computational model that accounts for the growth of liquid and/or solid reductant film growth on the interior of the exhaust system. The model determines reductant injection characteristics (e.g., amount, timing, etc.) that reduce and/or eliminate reductant films on the interior of the exhaust system. Exemplary inputs into the model include exhaust temperature, exhaust flow, and the amount of reductant injected in a previous injection cycle.

BACKGROUND

New air-pollution limits for diesel engines have caused somemanufacturers to adopt selective catalytic reduction (SCR) technologyfor reducing nitrogen oxides (NOx) in engine exhaust. The SCR processintroduces or injects an NOx reductant (e.g., a NOx reductant-watersolution) into the hot exhaust gas, which chemically reduces NOx intonon-pollutant compounds. When introduced, the NOx reductant-watersolution undergoes a decomposition process that releases ammonia intothe exhaust stream to facilitate NOx reduction in the SCR process.

One type of prior art diesel engine exhaust system that employs an SCRprocess is illustrated in FIG. 1. As best shown in FIG. 1, a dieselengine 4 produces NOx as a component of an exhaust stream. The exhauststream is directed from an exhaust port (not shown) of the engine 4 to adoser or injector section 8 through an exhaust pipe 10. The doser orinjector section 8 includes a NOx reductant doser or injector 12 that isconfigured to inject a solution of NOx reductant (or other nitrogenoxides reductant) into the exhaust stream present in the doser orinjector section 8.

After the NOx reductant doser or injector 12 injects NOx reductant intothe exhaust stream, the exhaust travels through an exhaust pipe 14 to acatalytic section 16, where the NOx reductant and a catalyst within thecatalytic section 16 react to reduce the nitrogen oxides intonon-pollutant species. The reduced nitrogen oxides and remaining exhaustcomponents are then directed out of the vehicle through an exhaustoutlet pipe 18.

However, during typical operation of an engine, the conditions for theSCR decomposition process are not optimal (e.g., exhaust is too hot orcold) and either too much, or too little, NOx reductant solution isintroduced into the exhaust. When introduced or injected NOx reductantsolution fails to undergo the SCR decomposition process (e.g., too muchNOx reductant solution for the exhaust temperature), NOx reductantcrystals will accumulate within the exhaust system, both on the interiorsurface of the exhaust pipe and at the port that introduces the solutioninto the exhaust. Build up of NOx reductant crystals in the exhaustsystem detrimentally affects the performance of the exhaust system, andalso is indicative of inefficiency in the SCR process: NOx reductantcrystals represent both wasted NOx reductant solution and reduced SCRefficiency.

During injection of NOx reductant into the diesel exhaust stream as partof the SCR process there is a significant amount of the total NOxreductant injected that contacts the walls of the exhaust pipe andbecomes a liquid wall film. While this process occurs, the NOx reductantthat is wetting the pipe walls does not reach the catalyst for itsintended use and the intended quantity of reductant is not available inthe catalyst.

Prior attempts at avoiding such “wall wetting” NOx reductant build uphave been only moderately successful. The primary method for avoidingNOx reductant build up is the use of a limiting table whereby a NOxreductant injector limits NOx reductant injection to a predeterminedamount based on average engine conditions as defined by a lookup-table.The limiting table is primarily focused on preventing NOx reductantbuild up and, thus, the resulting SCR performance is compromised becausethe amount of NOx reductant reaching the catalyst is typically less thanis required for optimal SCR performance.

Therefore, a system and method is desired for optimizing the injectionof NOx reductant solution into the exhaust of an operating diesel enginesuch that the SCR catalyst is provided sufficient NOx reductant whileNOx reductant build up prior to the catalyst is minimized.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In accordance with aspects of the present disclosure, a method fordetermining an amount of a reductant solution for nitrogen oxides to beinjected by an injector into an exhaust flow, from an engine producingnitrogen oxides, towards a catalyst, is provided. The method comprisesthe steps of: determining a first value of a first operating parameter;determining a first value of a second operating parameter; determining afirst injector flow rate of the reductant solution injected into theexhaust flow by the injector; determining a first reductant solutionpuddle size, wherein the first reductant solution puddle size isdetermined using the determined first value of the first operatingparameter, the determined first value of the second operating parameter,and the first injector flow rate; determining a reductant solutionpuddle delta, wherein the reductant solution puddle delta is determinedusing the determined first value of the first operating parameter, thedetermined first value of the second operating parameter, the firstinjector flow rate, and the first reductant solution puddle size; anddetermining a second injector flow rate of the reductant solution usingthe reductant solution puddle delta and the first injector flow rate ofthe reductant solution.

In accordance with another aspect of the present disclosure, acomputer-implemented method is provided for determining an amount of areductant solution for nitrogen oxides to be injected by an injectorinto an exhaust flow, from an engine producing nitrogen oxides, towardsa catalyst. The method comprises the steps of: receiving a first valueof a first operating parameter input; receiving a first value of asecond operating parameter input; receiving a first injector flow rateinput; determining a first reductant solution puddle size, wherein thefirst reductant solution puddle size is determined using the first valueof the first operating parameter input, the first value of the secondoperating parameter input, and the first injector flow rate input;determining a reductant solution puddle delta, wherein the reductantsolution puddle delta is determined using the first value of the firstoperating parameter input, the first value of the second operatingparameter input, the first injector flow rate input, and the firstreductant solution puddle size; and determining a second injector flowrate of the reductant solution using the reductant solution puddle deltaand the first injector flow rate input.

In accordance with yet another aspect of the present disclosure, asystem is provided for injecting a determined amount of a reductantsolution for nitrogen oxides into an exhaust flow from an engineproducing nitrogen oxides. The system comprises: an injector configuredto inject the reductant solution into the exhaust flow according to afirst injector flow rate; a first sensor configured to measure a firstoperating parameter of the exhaust flow; a second sensor configured tomeasure a second operating parameter of the exhaust flow; and a wallwetting controller. The wall wetting controller is configured to:determine a first reductant solution puddle size, wherein the firstreductant solution puddle size is determined using measured values ofthe first operating parameter, the second operating parameter, and thefirst injector flow rate; determine a reductant solution puddle delta,wherein the reductant solution puddle delta is determined using thefirst operating parameter, the second operating parameter, the firstinjector flow rate, and the first reductant solution puddle size; andtransmit a first control signal to the injector, wherein the firstcontrol signal is determined by the reductant solution puddle delta andthe first injector flow rate, and wherein the first control signaladjusts the first injector flow rate to a second injector flow rate.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of theclaimed subject matter will become more readily appreciated by referenceto the following detailed description, when taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a prior art exhaust system;

FIG. 2 is a diagrammatic illustration of one embodiment of an exhaustsystem for controlling injection of a nitrogen oxides reductant solutioninto the exhaust system in accordance with aspects of the presentdisclosure;

FIG. 3 is a schematic diagram of one exemplary embodiment of acontroller formed in accordance with aspects of the present disclosure;

FIG. 4 is a schematic diagram of an embodiment of a method forcontrolling injection of a nitrogen oxides reductant solution into anexhaust system in accordance with aspects of the present disclosure;

FIG. 5 is a schematic diagram of another embodiment of a method forcontrolling injection of a nitrogen oxides reductant solution into anexhaust system in accordance with aspects of the present disclosure; and

FIG. 6 is a schematic diagram of yet another embodiment of a method forcontrolling injection of a nitrogen oxides reductant solution into anexhaust system in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure will now be described withreference to the drawings, where like numerals correspond to likeelements. Embodiments of the present disclosure are generally directedto systems and methods for controlling the amount and/or timing ofnitrogen oxides (NOx) reductant (e.g., NOx reductant, used as anexemplary embodiment herein) injected into an exhaust system of a dieselpowered vehicle, such as a Class 8 truck. By controlling the amountand/or timing of NOx reductant injected into the exhaust system, areduction in the detrimental formation of NOx reductant films, bothliquid and solid, on interior surfaces of the exhaust system may beachieved. More particularly, embodiments of the present disclosure aredirected to methods and systems that determine, using one or moremeasured characteristics or operating parameters of the exhaust system(e.g., exhaust temperature, flow rate, etc.), the amount of NOxreductant to be injected into the exhaust system so as to balance theneeds of the SCR system for reducing NOx produced by the engine whilereducing NOx reductant film formation.

Embodiments of the present disclosure utilize one or more models thatdetermine the amount of NOx reductant to be injected into an exhaustsystem at a given time. In several embodiments of the presentdisclosure, the amount of NOx reductant to be injected into an exhaustsystem is based on measured or sensed conditions within the system andpredictive calculations of the amount of unused NOx reductant existingin the system (e.g., as a film on the interior walls of the exhaustsystem, etc.). One goal of the present disclosure among others is toreduce build up of liquid and solid NOx reductant on the surfaces of theexhaust system by limiting NOx reductant injection when the modelpredicts that excess NOx reductant is building in the exhaust system.

It should also be understood that several sections of the followingdescription regarding models, controllers or other computing devices forimplementing the models, etc., are presented largely in terms of logicand operations that may be performed by conventional electroniccomponents. These electronic components, which may be grouped in asingle location or distributed over a wide area, may generally includeprocessors, memory, input devices (e.g., sensors, etc.), etc. It will beappreciated by one skilled in the art that the logic described hereinmay be implemented in a variety of configurations, including software,hardware, or combinations thereof. The hardware may include but is notlimited to, analog circuitry, digital circuitry, processing units,application specific integrated circuits (ASICs), and the like. Incircumstances where the components are distributed, the components areaccessible to each other via communication links.

Although exemplary embodiments of the present disclosure will bedescribed hereinafter with reference to over-the-road vehicles,particularly diesel-powered, heavy-duty trucks, it will be appreciatedthat aspects of the present disclosure have wide application, andtherefore, may be suitable for use with many other types of vehiclesthat include a urea or other nitrogen-oxides reductant injection systemfor treating nitrogen oxides emissions in an exhaust stream, such asautomobiles, recreational vehicles, boats, etc. Examples of the subjectmatter of the present disclosure may find other applications, such asthe treatment of exhaust streams of stationary or portable generators,etc.

Accordingly, the following descriptions and illustrations herein shouldbe considered illustrative in nature, and thus, not limiting the scopeof the claimed subject matter.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of exemplary embodiments ofthe present disclosure. It will be apparent to one skilled in the art,however, that many embodiments of the present disclosure may bepracticed without some or all of the specific details. In someinstances, well-known process steps have not been described in detail inorder to not unnecessarily obscure various aspects of the presentdisclosure. Further, it will be appreciated that embodiments of thepresent disclosure may employ any combination of features describedherein.

Various embodiments of the present disclosure employ computationalmodels for at least one of the formation of liquid NOx reductant (the“Wall Wetting Model” 90) and solid NOx reductant (the “CrystallizationModel” 150) films within an exhaust system, such as a vehicle exhaustsystem. It will be appreciated that the development of NOx reductantfilms on a surface of an exhaust system may depend on several factors,including but not limited to the amount (and timing) of NOx reductantinjected into the system by the NOx reductant doser or injector, thetemperature of the exhaust gas, the rate of flow of the exhaust gas,exhaust pipe diameter, exhaust pipe surface area, ambient airtemperature (outside of pipes), NOx reductant evaporation time constant,exhaust water content (i.e., humidity or DewPoint), vehicle speed(airflow over exhaust system), etc. As will be described in more detailbelow, the models 90 and 150 may utilize the measured or sensed valuesof one or more of such factors in order to alter the control parameters(e.g., amount, timing, etc.) of a NOx reductant doser or injector of aSCR system for reducing NOx reductant film formation on inner surfacesof the exhaust system.

Turning now to FIG. 2, there is shown a partial view of an exemplary SCRsystem, designated 200, suitable for use with an exhaust system of avehicle, such as a class 8 diesel powered truck. As best shown in FIG.2, the SCR system 200 includes a doser or injector 212 associated withan exhaust system section 208, also known as a doser or injector section208, of the exhaust system. The exhaust system section 208 receives anexhaust gas stream from a source of NOx 218 (e.g., the vehicle's dieselengine) via an exhaust pipe 210. As the exhaust stream passes throughthe exhaust system section 208, the doser or injector 212 injects an NOxreductant into the stream of exhaust gas. After passing through theexhaust system section 208, the exhaust stream (e.g., treated with NOxreductant) is transported further “downstream” by exhaust pipe 214 to acatalyst, etc. (not shown). As shown in FIG. 2, the exhaust systemsection 208 defines one or more inner surfaces 220 onto which an amountof NOx reductant may form.

In the embodiment shown in FIG. 2, the SCR system 200 further includes acontroller 202 for implementing the NOx reductant film-formation models(e.g., Wall Wetting Model 90 and Crystallization Model 150). In oneembodiment, the controller 202 may include any suitable component ordevice and/or combinations thereof, that is capable of receiving inputsignals from one or more sensors, etc., processing and/or storing theinput signals, retrieving data from memory or other systems of thevehicle, and generating appropriate control signals for output to theinjector 212.

As used herein, controllers, control units, control modules, programmodules, etc., can contain logic for carrying out general or specificoperational features of the present disclosure. The logic can beimplemented in hardware components, such as analog circuitry, digitalcircuitry, processing units, or combinations thereof, or softwarecomponents having instructions which can be processed by the processingunits, etc. Therefore, as used herein, the term “controller” or“controlling component” can be used to generally describe theseaforementioned components, and can be either hardware or software, orcombinations thereof, that implement logic for carrying out variousaspects of the present disclosure.

The SCR system 200 further includes one or more sensors 204 associatedwith, for example, the exhaust system section 208. In one embodiment,the one or more sensors 204 may include but are not limited to atemperature sensor 204A, an exhaust flow rate sensor 204B, and anoptional NOx reductant flow sensor 204C. The sensors 204 provide inputsignals to the controller 202 for utilization by the NOx reductantfilm-formation models (e.g., Wall Wetting Model 90 and CrystallizationModel 150). The controller 202 additionally outputs control signals tothe NOx reductant injector 212 based on the results of the NOx reductantfilm-formation models (e.g., Wall Wetting Model 90 and/orCrystallization Model 150) for controlling the amount and/or timing ofNOx reductant to be injected by the NOx reductant injector 212 in thenext injection cycle.

It will be appreciated that the controller 202 and any one of thevarious sensors, etc., herein described may contain logic rulesimplemented for carrying out various aspects of the disclosed subjectmatter. To that end, one suitable example of the controller 202 is shownin FIG. 3. As best shown in FIG. 3, the controller 202 includes aprocessor 222 and memory 224 with a Random Access Memory (“RAM”), anElectronically Erasable, Programmable, Read-Only Memory (“EEPROM”) andany other suitable data storage means. Stored as executable instructionsin memory are program modules, which can include routines, programs,objects, components, data structures, etc., that perform particulartasks or implement particular abstract data types. In one embodiment,the program modules may include but are not limited to the models 90 andmodel 150.

The controller 202 is connected by an input/output (I/O) interface 226to various sensors, such as temperatures sensor 204A, exhaust flowsensor 204B, and optional NOx reductant flow sensor 204C. Other systems,devices and/or controllers of the vehicle not illustrated but known inthe art, such as an engine control unit (ECU), transmission control unit(TCU), etc., can also be connected to the interface 226 via avehicle-wide network 240 or other communication link. In that regard,the controller 202 may receive other vehicle or system data, such asvehicle speed data, engine speed data, fuel consumption data, humidityand/or Dew point data, ambient temperature data external to the exhaustsystem, etc.

Returning to FIG. 2, the temperature sensor 204A, the exhaust flowsensor 204B and the optional NOx reductant flow sensor 204C measure orsense an exhaust system condition and transmit signals indicative ofsuch exhaust system conditions to the controller 202. The temperaturesensor 204A measures the temperature of exhaust gas passing through theexhaust system section 208 at the location of the temperature sensor204A. The temperature sensor 204A can be any temperature sensor known tothose of skill in the art. An exemplary temperature sensor 204A usefulin the system is a thermocouple. The temperature sensor 204A isoperatively connected to the controller 202 to deliver a signalindicating the temperature measured in the exhaust system at thelocation of the temperature sensor 204A at a given time.

The exhaust flow sensor 204B of the system 200 measures the flow ofexhaust gas passing through the exhaust system section 208 at thelocation of the exhaust flow sensor 204B. The exhaust flow sensors 204Bcan be any exhaust flow sensor known to those of skill in the art. Theexhaust flow sensor 204B is operatively connected to the controller 202to deliver a signal indicating the exhaust flow at the location of theexhaust flow sensor 204B at a given time.

The optional NOx reductant flow sensor 204C is used to measure the flowof NOx reductant from the NOx reductant injector 212 passing into theexhaust system section 208. The NOx reductant flow sensor 204C can beany liquid flow sensor known to those of skill in the art. The NOxreductant flow sensor 204C is operatively connected to the controller202 to deliver a signal indicating the flow of NOx reductant injectedfrom the injector 212 at a given time.

It will be appreciated that the NOx reductant injector 212 in someembodiments does not inject NOx reductant in a continuous stream, butinstead injects in cycled bursts depending on the NOx reductant demandindicated by the controller 202. Thus, for example, if the controller202 indicates that more NOx reductant is needed in the system 200 as aresult, for example, of exhaust gas temperature and/or flow, thecontroller 202 will instruct the NOx reductant injector 212 viaappropriate control signals to inject an amount of NOx reductantappropriate for the conditions within the exhaust system. However,because the amount of NOx reductant the controller 202 instructs the NOxreductant injector 212 to inject is not always the same as theactually-delivered amount of NOx reductant, the NOx reductant flowsensor 204C can be used to increase the accuracy of the provided models90 and 150 and to provide a more accurate system for reducing built-upNOx reductant films in the exhaust system.

Referring now to FIG. 4, a schematic diagram of an exemplary WallWetting model 90 of the present disclosure is illustrated. The WallWetting model 90 implements a feed-forward adjustment of NOx reductantsolution injected during an SCR process. In the described embodiments,the terms urea and NOx reductant are used interchangeably, althoughother NOx reductants can be used. Generally described, the Wall WettingModel 90 predicts a current amount of NOx reductant 100, referred to asthe urea puddle size, that will wet the interior surface 220 of theexhaust system section 208. The model 90 then utilizes several measuredor sensed conditions or operating parameters, for example, of theexhaust system to determine a change (either positive or negative) inthe NOx reductant solution flow, referred to herein as the NOx reductantpuddle delta, in order to reduce (or eliminate) the NOx reductant liquidwetting of the interior surface 220, while still supplying sufficientNOx reductant to the SCR process. These conditions or parameters mayinclude but are not limited to the temperature of the exhaust gas, therate of flow of the exhaust gas, exhaust pipe diameter, exhaust pipesurface area, ambient air temperature (outside of pipes), NOx reductantevaporation time constant, exhaust water content (i.e., humidity orDewPoint), vehicle speed (airflow over exhaust system), etc. In otherwords, the model 90 attempts to balance the need for NOx reductant inthe SCR process and the desire to reduce and eliminate NOx reductantbuild up on the interior surface 220 of the exhaust system.

As implemented in several embodiments, the Wall Wetting Model 90 is usedto determine the change in the amount of NOx reductant injected bychanging a first injector flow rate to a modified or second injectorflow rate as a result of determining the NOx reductant puddle delta.Further iterations of the Wall Wetting Model 90 then adjust the secondinjector flow rate to a third injector flow rate, and so on.

The Wall Wetting Model 90 will now be described in more detail. In theembodiment shown in FIG. 4, the Wall Wetting Model 90 utilizes exhaustsystem parameters, although other types and/or numbers of variables maybe employed. For example, in one embodiment, the Wall Wetting Model 90utilizes NOx reductant dosed 94, exhaust temperature 92, and exhaustflow 98. The exhaust temperature 92 and exhaust flow 98 inputted intothe Wall Wetting Model 90 are provided by the temperature sensor 204A(FIG. 2) and the exhaust flow sensor 204B (FIG. 2), respectively.

The NOx reductant dosed 94, in one embodiment, is indicative of a NOxreductant demand that can be a predetermined value representing theamount of NOx reductant (a first injector flow rate) that the NOxreductant injector 212 is instructed to deliver (e.g., by controller202). Alternatively, since the amount of NOx reductant instructed to bedelivered and the amount actually delivered can be different, otherembodiments may input the actual urea dosed into the Wall Wetting Model90 (as the urea dosed 94 variable) as determined by the NOx reductantflow sensor 204C (see FIGS. 5 and 6), instead of the NOx reductantdemand.

Referring again to FIG. 4, once inputted into the model 90, the ureadosed 94, the exhaust temperature 92 and the exhaust flow 98 are used todetermine a NOx reductant puddle size 100 and a NOx reductant puddledelta 96. For example, the NOx reductant puddle size 100 is a calculatedprediction of the amount of liquid NOx reductant formed on the interiorsurface 220 at a particular time. The NOx reductant puddle size 100 isdetermined using NOx reductant dosed 94, exhaust temperature 92, andexhaust flow 98. The Wall Wetting Model 90 calculates the NOx reductantpuddle size 100 using equations for modeling the dynamics of liquidbehavior known to those of skill in the art.

On the other hand, the NOx reductant puddle delta 96 is the change(either increase or decrease) in the NOx reductant demand determined bythe Wall Wetting Model 90 that will reduce and/or eliminate the liquidNOx reductant determined to exist in the exhaust system according to thecalculated NOx reductant puddle size 100. The NOx reductant puddle delta96 may also indicate that no change is needed to the NOx reductantdemand of the present injection cycle. The NOx reductant puddle delta 96is determined, using modeling methods known to those of skill in theart, by the Wall Wetting Model 90 using the NOx reductant dosed variable94, the exhaust temperature variable 92, the exhaust flow variable 98,in conjunction with the determined value of the NOx reductant puddlesize 100.

One purpose of the Wall Wetting Model 90 is to determine the adjustmentto future NOx reductant injection cycles that will act to reduce thewetting of NOx on the surface of the exhaust system injector section208. This adjustment to the amount of NOx to be injected is illustratedin FIG. 4 as the adjusted NOx reductant demand 80. The adjusted NOxreductant demand 80 is determined by combining at addition point 98 theNOx reductant puddle delta 96 calculated by the Wall Wetting Model 90with the predetermined NOx reductant demand. The adjusted NOx reductantdemand 80, as a result of the combination of the NOx reductant puddledelta 96 and the NOx reductant demand, is then utilized by thecontroller to generate and/or transmit appropriate control signals tothe injector 212. Therefore, the adjusted NOx reductant demand 80 is theamount of NOx reductant determined by the Wall Wetting Model 90 to beinjected in an injection cycle by the NOx reductant injector 212 intothe exhaust system to satisfy the SCR system requirements for amount ofNOx reductant needed, as well as to reduce the formation of liquid NOxreductant on interior surfaces 220 of the exhaust system.

It will be appreciated that the adjusted NOx reductant demand 80 is alsoemployed by the Wall Wetting Model 90 as the NOx reductant dosedvariable 94 for the next cycle. Thus, the cyclical nature of the WallWetting Model 90 of FIG. 4 is illustrated: the NOx reductant dosedvariable 94 is used to calculate the NOx reductant puddle delta 96,which then affects the NOx reductant dosed variable 94 when combinedwith the NOx reductant demand 50 input. Further, the adjusted NOxreductant demand 80 then becomes the NOx reductant demand for the nextcycle of the method, which will determine the next adjusted NOxreductant demand 80 required by the system, and so on.

As described above, the Wall Wetting Model 90 can be executed andimplemented in the controller 202, although the Wall Wetting Model 90can also be implemented in a computational logic device, as known tothose of skill in the art (e.g., a computer, an applications-specificintegrated circuit (ASIC), a field-programmable gate array (FPGA),etc.), separate from the controller 202. The results of the model (i.e.,adjusted NOx reductant demand 80, 120, or 156) may then be subsequentlytransmitted to the controller 202 for processing and control of theinjector 212. Similarly, the Crystallization Model 150 can also beintegrated into the controller 202 or housed in a separate logic device(not illustrated).

Turning now to FIG. 5, there is illustrated another embodiment of thepresent disclosure. In this embodiment, the Wall Wetting Model 90, asdescribed above, is used in a system 400 having the same outputs, NOxreductant puddle size 100 and NOx reductant puddle delta 96, butmodified inputs compared to the system 300 of FIG. 4. In that regard,the NOx reductant dosed variable 94 of the Wall Wetting Model 90 isdetermined by an actual NOx reductant dosed input instead of a NOxreductant demand input. The difference between the actual NOx reductantdosed and the NOx reductant demand is that the actual NOx reductantdosed is based on the NOx reductant amount actually injected by the NOxreductant injector 212, as measured by a NOx reductant flow sensor 204C(see FIG. 2). On the other hand, the NOx reductant demand is an idealvalue that represents the amount of NOx reductant that the NOx reductantinjector 212 is instructed to inject (e.g., by controller 202); however,due to various failure mechanisms (e.g., mechanical failure of the NOxreductant injector 212), the actual amount of NOx reductant dosed doesnot always match the instructed amount of NOx reductant to be injected.Thus, by using the NOx reductant flow sensor 204C and incorporating theoutput from the sensor 204C (i.e., the actual NOx reductant dosed) intothe Wall Wetting Model 90 as the NOx reductant dosed variable 94, theWall Wetting Model 90 provides for a more accurate overall determinationof the NOx reductant puddle size 100 and NOx reductant puddle delta 96resulting from the Wall Wetting Model 90.

In yet another embodiment of the present disclosure, a system 500 mayalso implement a Crystallization Model 150 in addition to the WallWetting Model 90, as illustrated in the schematic diagram of FIG. 6. Inthe system 500, the Crystallization Model 150 takes into account thepossibility that liquid NOx reductant adhered to an interior surface 220of the exhaust system 200 will crystallize into a solid NOx reductantfilm under certain conditions (e.g., temperature and exhaust flowconditions).

To that end, the Crystallization Model 150 calculates the amount of NOxreductant to be injected (referred to as adjusted NOx reductant demand156) to optimally remove both solid and liquid NOx reductant films fromthe exhaust system 200 while maintaining the demand for NOx reductant ofthe SCR system. Solid NOx reductant is removed by adjusting the exhaustconditions (e.g., temperature and/or NOx reductant amount injected.etc.) such that solidified NOx reductant is re-liquefied so that it canbe transported in the exhaust to its intended destination of thecatalyst.

The Crystallization Model 150 utilizes a number of variables tocalculate the adjusted NOx reductant demand 156. The CrystallizationModel 150 calculates the adjusted NOx reductant demand 156 usingequations for modeling the dynamics of solid/liquid behavior known tothose of skill in the art. Similar to the Wall Wetting Model 90, theCrystallization Model 150 uses exhaust temperature 158 and exhaust flow154, provided by the exhaust temperature sensor 204A and the exhaustflow sensor 204B, respectively. The Crystallization Model 150additionally includes a NOx reductant puddle 152 variable, which isdetermined by the Wall Wetting Model 90 as the NOx reductant puddle size100. Finally, the Crystallization Model 150 uses a NOx reductant demand148 variable, which is determined by combining the NOx reductant puddledelta 96 output by the Wall Wetting Model 90 with the NOx reductantdemand at addition point 98 to provide the NOx reductant demand 148.Therefore, the NOx reductant demand variable 148 of the system 500 issimilar to the adjusted NOx reductant demand 120 determined by thesystem 400 illustrated in FIG. 5 (which does not take into account theCrystallization Model 150).

The Crystallization Model 150 calculates an adjusted NOx reductantdemand 156, which is then outputted as the adjusted NOx reductant demandoutput. Similar to the adjusted NOx reductant demand 80 (as illustratedin FIG. 4) and the adjusted NOx reductant demand 120 (as illustrated inFIG. 5), the adjusted NOx reductant demand 156 illustrated in FIG. 6 isthe calculated amount of NOx reductant to be injected by the NOxreductant injector 212 in a future injection cycle. Essentially, theadjusted NOx reductant demand 80, 120, or 156 then becomes the NOxreductant demand variable for the next iteration of the method (i.e.,the next injection cycle).

It will be appreciated that the Wall Wetting Model 90 andCrystallization Model 150 need not be limited to calculating adjustedurea demand based on modeling specific to the interior surface 220 ofthe exhaust system section 208, as illustrated in FIG. 2 and describedherein. The models 90 and/or 150 can calculate adjusted urea demandsbased on modeled conditions throughout the exhaust system, includingmultiple locations in the same model. For example, adjusted urea demandcan be determined in relation to the exhaust pipe, the catalyst (notpictured), or an exhaust mixer (not pictured) intermediate the catalystand exhaust pipe.

Furthermore, when calculating adjusted urea demand, the models 90 and150 can each incorporate multiple locations throughout the exhaustsystem. For example, the crystallization model 150 can calculate theurea demand (using the same inputs as in FIG. 6) by calculating ureacrystallization in the section 208, exhaust pipe, and exhaust mixer (notpictured). By incorporating several locations into the models 90 and150, adjusted urea injection conditions can be determined that reduceurea wetting and crystallization throughout the exhaust system insteadof at just a single location (e.g., the section 220).

While the Wall Wetting Model 90 and Crystallization Model 150 areillustrated separately in FIG. 6, it will be appreciated that the twomodels can be combined in a single model. Additionally, while theembodiment illustrated in FIG. 6 incorporates actual NOx reductant dosedas an input to the Wall Wetting Model 90, NOx reductant demand can beused instead of actual NOx reductant dosed, such as in the embodimentillustrated in FIG. 4. However, using actual NOx reductant dosed as aninput will typically provide more accurate modeling of the NOx reductantwithin the exhaust system and a more effective adjusted NOx reductantdemand 156 output, if desired.

To implement the methods provided herein, the appropriate input andoutput controls are incorporated into the exhaust system. For example,the temperature and exhaust flow are determined (e.g., measured) tobegin the Wall Wetting Model 90 and crystallization model 150.Alternatively, the temperature and exhaust flow can be estimated;however, actual measurements may result in more accurate adjusted NOxreductant demand 80, 120, 156 outputs.

The system and methods in accordance with the embodiments of the presentdisclosure can be built into a production vehicle during assembly (e.g.,on an assembly line), or can be added as an aftermarket modification toan already-built vehicle. For example, an aftermarket modification ofthe vehicle may include installing a temperature sensor 204A and anexhaust flow sensor 204B in the exhaust system, as well as installing acontroller 202 (or adapting a pre-existing onboard controller) tointerface with the temperature sensor 204A, exhaust flow sensor 204B andNOx reductant injector 212 (and optionally a NOx reductant sensor 204C).Additionally, the installed controller or existing onboard controllercould be configured to implement the Wall Wetting Model 90 and/orCrystallization Model 150 in accordance with the embodiments describedherein.

The principles, representative embodiments, and modes of operation ofthe present disclosure have been described in the foregoing description.However, aspects of the present disclosure which are intended to beprotected are not to be construed as limited to the particularembodiments disclosed. Further, the embodiments described herein are tobe regarded as illustrative rather than restrictive. It will beappreciated that variations and changes may be made by others, andequivalents employed, without departing from the spirit of the presentdisclosure. Accordingly, it is expressly intended that all suchvariations, changes, and equivalents fall within the spirit and scope ofthe present disclosure, as claimed.

The embodiments of the present disclosure in which an exclusive propertyor privilege is claimed are defined as follows:
 1. A method of using anon-board vehicle computing system to determine an amount of a reductantsolution for reacting with nitrogen oxides, the reductant solution to beinjected by an injector into an exhaust flow from an engine producingnitrogen oxides towards a catalyst, the method comprising: (a)determining a first value of a first operating parameter using theon-board vehicle computing system; (b) determining a first value of asecond operating parameter using the on-board vehicle computing system;(c) determining a first injector flow rate of the reductant solutioninjected into the exhaust flow by the injector using the on-boardvehicle computing system; (d) determining a first reductant solutionpuddle size using the on-board vehicle computing system using thedetermined first value of the first operating parameter, the determinedfirst value of the second operating parameter, and the first injectorflow rate; (e) determining a reductant solution puddle delta using theon-board vehicle computing system using the determined first value ofthe first operating parameter, the determined first value of the secondoperating parameter, the first injector flow rate, and the firstreductant solution puddle size; (f) determining a second injector flowrate parameter of the reductant solution using the on-board vehiclecomputing system, wherein the second injector flow rate parameter isdetermined using the determined reductant solution puddle delta and thedetermined first injector flow rate of the reductant solution; (g)determining a third injector flow rate of the reductant solution usingthe on-board vehicle computing system, wherein the third injector flowrate is determined using the determined first value of the firstoperating parameter, the determined first value of the second operatingparameter, the determined first reductant solution puddle size, and thedetermined second injector flow rate parameter; and (h) adjusting thefirst injector flow rate of the reductant solution injected by theinjector into the exhaust flow to the third injector flow rate of thereductant solution.
 2. The method of claim 1, wherein the first injectorflow rate is a first demand flow rate, wherein the first demand flowrate is a predetermined amount of the reductant solution to be injectedinto the exhaust flow.
 3. The method of claim 1, wherein the firstinjector flow rate is a first actual flow rate, wherein the first actualflow rate is the actual amount of the reductant solution injected by theinjector into the exhaust flow.
 4. The method of claim 1 wherein thefirst operating parameter and the second operating parameter areindependently selected from the group consisting of an exhausttemperature, an exhaust flow, an exhaust pipe diameter, an exhaust pipesurface area, an ambient air temperature, a reductant evaporation timeconstant, an exhaust water content, and a vehicle speed.
 5. Anon-transitory computer-readable medium having computer-executableinstructions stored thereon that, in response to execution by one ormore on-board vehicle computing devices, cause the one or more on-boardcomputing devices to: determine an amount of a reductant solution forreacting with nitrogen oxides, the reductant solution to be injected byan injector into an exhaust flow from an engine producing nitrogenoxides towards a catalyst, the one or more computing devices using thesteps: (a) receive a first value of a first operating parameter input;(b) receive a first value of a second operating parameter input; (c)receive a first injector flow rate input; (d) determine a firstreductant solution puddle size using the first value of the firstoperating parameter input, the first value of the second operatingparameter input, and the first injector flow rate input; (e) determine areductant solution puddle delta using the first value of the firstoperating parameter input, the first value of the second operatingparameter input, the first injector flow rate input, and the firstreductant solution puddle size; (f) determine a second injector flowrate parameter of the reductant solution using the determined reductantsolution puddle delta and the first injector flow rate input; (g)determine a third injector flow rate using the first value of the firstoperating parameter input, the first value of the second operatingparameter input, the determined reductant solution puddle size, and thedetermined second injector flow rate parameter; and (h) adjust theamount of the reductant solution injected by the injector to the thirdinjector flow rate.
 6. The computer-readable medium of claim 5, whereinthe first injector flow rate input is an actual first flow rate, whereinthe actual first flow rate is a measured amount of the reductantsolution injected into the exhaust flow.
 7. The computer-readable mediumof claim 5, wherein the first injector flow rate input is a first demandflow rate input, wherein the first demand flow rate input is apredetermined amount of the reductant solution to be injected into theexhaust flow.
 8. The computer-readable medium of claim 5 further causingthe one or more computing devices to adjust the first injector flow rateto the third injector flow rate.
 9. The computer-readable medium ofclaim 8, wherein the first operating parameter and the second operatingparameter are independently selected by the one or more computingdevices from the group consisting of an exhaust temperature, an exhaustflow, an exhaust pipe diameter, an exhaust pipe surface area, an ambientair temperature, a reductant evaporation time constant, an exhaust watercontent, and a vehicle speed.
 10. A system for injecting a determinedamount of a reductant solution for reacting with nitrogen oxides into anexhaust flow from an engine producing nitrogen oxides, the systemcomprising: (a) an injector configured to inject the reductant solutioninto the exhaust flow according to a first injector flow rate; (b) afirst sensor configured to measure a first operating parameter of theexhaust flow; (c) a second sensor configured to measure a secondoperating parameter of the exhaust flow; (d) a wall wetting controllerconfigured to: (i) determine a first reductant solution puddle size,wherein the first reductant solution puddle size is determined usingmeasured values of the first operating parameter, the second operatingparameter, and the first injector flow rate; (ii) determine a reductantsolution puddle delta, wherein the reductant solution puddle delta isdetermined using the first operating parameter, the second operatingparameter, the first injector flow rate, and the determined firstreductant solution puddle size; (iii) determine a second injector flowrate using measured values of the first operating parameter and thesecond operating parameter, and the determined values of the firstreductant puddle size and the first injector flow rate; and (iv)transmit a first control signal to the injector, wherein the firstcontrol signal is determined by the reductant solution puddle delta andthe first injector flow rate, and wherein the first control signaladjusts the first injector flow rate to the second injector flow rate,and (e) a crystallization controller configured to: (i) determine athird injector flow rate using measured values of the first operatingparameter and the second operating parameter, and the determined valuesof the first reductant puddle size and the second injector flow rate;and (ii) transmit a second control signal to the injector, wherein thesecond control signal is determined using the third injector flow rate,and wherein the second control signal adjusts the second injector flowrate to the third injector flow rate.
 11. The system of claim 10,wherein the first injector flow rate is a first demand flow rate,wherein the first demand flow rate is a predetermined amount of thereductant solution to be injected into the exhaust flow.
 12. The systemof claim 10 further comprising an injector flow rate sensor configuredto determine a measured first injector flow rate of the injector, andwherein the first measured injector flow rate is the first injector flowrate.
 13. The system of claim 10, wherein the first operating parameterand the second operating parameter are independently selected from thegroup consisting of an exhaust temperature, an exhaust flow, an exhaustpipe diameter, an exhaust pipe surface area, an ambient air temperature,a reductant evaporation time constant, an exhaust water content, and avehicle speed.
 14. The method of claim 1, wherein the reductant solutionis a urea solution.
 15. The computer-readable medium of claim 5, whereinthe reductant solution is a urea solution.
 16. The system of claim 10,wherein the reductant solution is a urea solution.